1. Field of the Invention
The present invention relates generally to an intake burner for use in internal combustion engines, such as diesel engines, for heating intake air flowing through an air intake system to enable the engine to start more smoothly and reliably. More particularly, the invention relates to an intake burner having an electric heater made of a ceramic material and having a heating resistor embedded therein.
2. Description of the Relevant Art
Internal combustion engines, particularly diesel engines, fail to start smoothly under low temperature conditions since the fuel may not be ignited when low temperature air is compressed in the engine cylinders.
To solve this problem, internal combustion engines have been equipped with air intake systems utilizing an intake burner. This is shown in FIG. 1 of the accompanying drawings in which the engine 1 has an air intake system 2 provided with an intake burner 3 which heats intake air flowing through the air intake system 2 by the burning of fuel in the intake air. This thereby improves the ability of the engine 1 to start smoothly and reliably under low air temperature conditions.
FIGS. 2 and 3 illustrate different types of known intake burners. The intake burner 3a in FIG. 2 is mounted on an intake pipe 4 and has a burner end 5 extending into an air intake system 2 for heating intake air flowing therein by burning supplied fuel. The intake burner 3a comprises a fuel feed tube 6 for feeding fuel supplied from a fuel pump (not shown) to the burner end 5, and a heating resistor 7 disposed around the fuel feed tube 6 for heating the fuel. The heating resistor 7 comprises a Nichrome wire coil that can be raised to a red heat on application of a voltage from a power supply (not shown). The fuel flowing from an inlet 6a of the fuel feed tube 6 is metered by an orifice 6b in passing through the fuel feed tube 6, and is then vaporized by being heated by the heating resistor 7. The vaporized fuel passes into the burner end 5 where the fuel vapor is ignited by further heating action of the heating resistor 7. Combustion of the fuel in the air intake system 2 thus heats air supplied into the engine 1.
The intake burner 3a shown in FIG. 2 has, however, suffered from the following problems:
(1) The heating resistor 7 comprises a Nichrome wire which is exposed for effective thermal radiation, and if a high voltage is applied to the heating resistor 7 to heat the latter to a red heat in order to start the engine 1 in a reduced period of time, the heating resistor 7 tends to be oxidized or corroded by sulfur oxides produced by combustion of sulfur in the fuel. The heating resistor 7 is thus liable to be broken and less durable in service. To avoid this, the heating resistor 7 has to be heated slowly, and the engine 1 therefor cannot be started quickly.
(2) The heating resistor 7 is directly exposed to low temperature intake air, and when the engine 1 is started by energizing a starting motor, the heating resistor 7 is cooled by the increased flow of intake air and may fail to burn the fuel, thus failing to start the engine 1 smoothly.
The intake burner shown in FIG. 3 is generally denoted at 3b and has an outer tube 8 serving as an outer casing attached to and extending into an air intake tube 4 and a heater 12 comprising a metal sheath 9 housing a heating resistor 10 in the form of a Nichrome wire coil. The sheath 9 also contains a volume of powder 11 such as magnesium oxide packed for increased thermal capacity. The heater 12 is heated upon energizing the heating resistor 10 by the application of a power supply voltage through a terminal 13 at an outer end of the outer tube 8.
The intake burner 3b also has a fuel feed nozzle 14 disposed outside of the air intake system 2 and connected to a fuel pump (not shown), the fuel feed nozzle 14 communicating with a vaporizing region 15. Fuel as fed from the fuel feed nozzle 14 is heated and vaporized in the vaporizing region 15, and the vaporized fuel is fed along an outer peripheral surface of the heater 12 in the longitudinal direction thereof. The intake burner 3b has a combustion region 16 extending into the air intake system 2 for additionally heating and burning of the vaporized fuel fed from the vaporizing region 15, to thereby heat the intake air. A tubular holder 17 is disposed in the outer tube 8 and surrounds the outer peripheral surface of the heater 12 longitudinally therealong. The holder 17 serves to fill the space between itself and the heater 12 with the fuel continuously supplied from the fuel feed nozzle 14 and to feed the fuel from the vaporizing region 15 to the combustion region 16 for promoting the vaporization of the fuel. A tubular sleeve 18 prevents flames of the burned fuel from being blown out by a high rate of flow of intake air.
The intake burner 3b as shown in FIG. 3 has also had shortcomings, as follows:
(1) The metal sheath 9 is heated indirectly by the heating resistor 10 through the volume of magnesium oxide powder 11, insulating the heating resistor 10 from the atmosphere. This construction requires a longer time to heat the heater 12, and hence cannot achieve a reduction in the time for starting the engine 1.
(2) Since the volume of powder 11 as of magnesium oxide is packed in the metal sheath 9, the coils of the heating resistor 10 may be mispositioned and may sometimes be short-circuited. This structural difficulty results in a failure to achieve adequate temperature control, and a desired temperature distribution cannot be accomplished over the zone from the vaporizing region 15 to the combustion region 16. Excessive combustion which may result from the foregoing shortcoming consumes too much oxygen in the air intake system 2 which then becomes short of oxygen. On the other hand, the heater 12 cannot be sufficiently heated as a whole, with the result that the fuel will not be ignited smoothly or the flames will die out. Accordingly, a smooth engine starting capability cannot be achieved.
With the foregoing problems of the prior art in view, the inventors have directed their attention to a ceramic material for use as an intake burner heater material due to its good heat, oxidation, and corrosion resistance. The inventors have found that the problems with the prior art intake burners can be solved by forming an entire heater 19 (shown in FIG. 4 of the accompanying drawings) of a ceramic material into a rod shape and embedding a heating resistor 20 such as a tungsten wire in the heater 19, the heater 19 being supported by a holder 21.
It is however necessary to take into account the following considerations in constructing such an intake burner:
(1) When fuel is supplied from a fuel feed nozzle (not shown), the vaporizing region 22 between the holder 21 and the heater 19 is cooled, and the resistance of the heating resistor 20 in the vaporizing region 22 is reduced due to the resulting temperature drop. Thus, only a partial voltage drop takes place along the heating resistor 20 in the vaporizing region 22. Although the combustion region 23 around the heater 19 extends out of the holder 21 and is therefore subjected to a low temperature air flow, the heating resistor 20 has applied to it an increased voltage due to the reduced voltage drop in the vaporizing region 22, and thus generates increased heat.
Where the heating resistor 20 is constructed with coils of equal pitch, the surface temperature T of the heater 19 is substantially uniform before fuel is supplied throughout the length of the heater 19, that is, from the vaporizing region 22 to the combustion region 23. This is shown by the dot-and-dash line A in FIG. 4. When fuel is fed into the vaporizing region 22, the latter is excessively cooled and fails to vaporize the fuel smoothly. At the same time, the heating resistor 20 in the combustion region 23 is unduly heated to a surface temperature higher than an allowable maximum temperature Tmax (shown by the solid line B) for the ceramic material of the heater 19. The heater 19 is then liable to get cracked due to the excessive heat or the heating resistor 20 tends to be broken.
Also designated in FIG. 4 is T.sub.F, the lowest surface temperature producing fuel ignition and T.sub.L, the lowest surface temperature producing proper fuel vaporization. If the voltage applied by a power supply 24 is lowered to eliminate the above shortcoming, then all of the surface temperatures T of the heater 19 would be lowered correspondingly, and the heater 19 in the vaporizing region 22 could be lowered to a temperature below T.sub.L, a temperature at which it would fail to vaporize the fuel. Similarly, any vaporized fuel might not be ignited due to the exposure of the combuation region 23 to flowing intake air, and resultant cooling of the heater 19 in that region below T.sub.F.
(2) Due to limitations (described later) on the shape of the heater 19, the heater 19 is substantially rectangular, preferably square, in cross section, with a large clearance space between the heater 19 and the holder 21 resulting from the cylindrical shape of the holder 21, as shown in FIGS. 5 and 6 of the accompanying drawings. When fuel is supplied from a fuel feed nozzle 25 onto the outer surface of the heater 19, the fuel does not at once fill the holder 21 during start up, and this results in substantially no fuel applied to a lower surface 19a of the heater 19 remote from the fuel feed nozzle 25. During start up, therefore, the lower heater surface 19a does not contribute to fuel vaporization. Since the lower heater surface 19a is widely spaced from an inner holder wall 21a below the heater 19, a pool of fuel is formed therebetween which will not be heated but will flow out from between the heater 19 and the holder 21 in the direction of the arrow C.
As a consequence, the fuel cannot be vaporized smoothly in the vaporizing region 22 surrounded by the holder 21 even if the heater performance is good, and also the fuel cannot be ignited instantaneously and reliably in the combustion region 23. It is desirable to eliminate the above problem for stable fuel combustion.
Another difficulty is that any unvaporized fuel is likely to flow out of an outer tube into the air intake tube 4 and be trapped therein, thereby damaging the air intake system 2.
(3) Where the holder 21 is shaped complementarily to the heater 19 to avoid the problem mentioned above in (2), the following problems must be solved: As shown in FIG. 7 of the accompanying drawings, a support member 26 of metal for attaching the heater 19 to the outer tube (not shown) is mounted on the heater 19 adjacent to the vaporizing region 22. The support member 26 has a hole 26a of a rectangular cross section (which may be square as shown), in which the heater 19 is fitted, and an outer cylindrical surface 26b adapted to be within a bore formed in the outer tube (not shown). The support member 26 is of a stepped configuration so as to be pushed and held in position by a bolt threaded in an end of the outer tube, as will be described.
As illustrated in FIG. 8, the holder 21 is formed so that the heater 19 may be covered by the holder 21 between the vaporizing region 22 and the combustion region 23 for promoting fuel vaporization. For this purpose, holder 21 has an inner surface 21b of a rectangular cross section spaced an equal distance from an outer peripheral surface 19b of the heater 19. The holder 21 also has an outer cylindrical surface 21c adapted to be fitted within a bore of the outer tube (not shown). The holder 21 has a fuel inlet 27 through which fuel is introduced from the fuel feed nozzle 25.
The holder 21 and the support member 26 have their inner surfaces 21b, 26a of a rectangular cross section (which may be square as shown) and their outer surfaces 21c, 26b of a cylindrical shape due to limitations on the configuration of the heater 19 and the ease of machining of the outer tube. Its inner cylindrical surface can also easily be machined by known techniques. Such holder 21 and support member 26 can be fabricated by machining a solid cylindrical body or a thick tubular member through discharge machining to the shape as shown in FIGS. 7 and 8. However, this type of machining process is highly complex and could not be relied upon for mass production. Furthermore, machined products would be too thick, result in an increased material cost, and increase their weight.